This invention relates generally to the field of machine vision systems and particularly to automated visual inspection in the semiconductor manufacturing process.
Integrated circuit manufacturers generally want to improve the quality of their manufacturing and reduce their costs. In applications such as wire bonding of semiconductor integrated circuits inspection is used to help accomplish this.
A human inspector can only inspect on a sampled basis after bonding has taken place. Automatic inspection can inspect the bond while it is still on the wirebonder machine perhaps allowing feedback to the machine to improve its operating parameters or allowing immediate rework of the part. Automatic inspection can be run exhaustively on all parts being manufactured while human inspection is too expensive to do so.
There are no previously known solutions to the problem of inspecting the wire bonds on the lead frame side automatically. The bonds are currently inspected manually by a person using a microscope (typically on a sampled basis). Using a person to inspect leads manually is both slow and expensive. Inspecting leads on a sampled basis may allow poorly bonded devices to slip through.
Theoretically, previously known automated binary boundary tracking techniques, such as those shown in FIG. 4 might be applied. A binary boundary tracker follows a grey level contour in an image. As shown in FIG. 4, image pixels whose grey value are greater than a threshold are on one side of the contour, these are pixels inside the object 42. Image pixels whose grey value are less than or equal to the threshold are on the other side of the contour and these are shown as background pixels 46. The contour is one pixel wide and runs between these two regions of image pixels, shown in FIG. 4 by a number of boundary pixels 44. A difficulty in using a binary boundary tracker is choosing a correct threshold value. If the threshold is chosen too low, the contour will move in one direction and the contoured region will be too small. If the threshold is chosen too high, the contour will move in the other direction and the contoured region will be too large. Any measurements that are based on the contour may be incorrect if the threshold is chosen incorrectly.
It is an object of this invention to automatically measure the size of the solder bond crescent produced by a wire bonder (or similar machine) on the lead side from imagery.
It is another object to determine the presence or absence of a wire extending from the crescent in the direction of the semiconductor die.